The present invention relates to an inline type color picture tube in which a coma distortion correcting mechanism to make a central electron beam and a pair of outside electron beams accurately coincide on a fluorescent screen and to eliminate a coma distortion is provided in electron guns.
In general, in what is called an inline type color picture tube in which three electron guns for generating three electron beams for red, green, and blue are arranged in a line in the horizontal direction, the magnetic field generated by a vertical deflecting coil is formed like a barrel as shown in FIG. 1 and the magnetic field generated by a horizontal coil is formed like a pincushion shape as shown in FIG. 2, so that rasters as shown in FIG. 3 can be obtained on the screen of a color picture tube. Namely, in FIGS. 1 and 2, reference numerals 1, 2, and 3 denote red, green, and blue electron beams; 4 indicates a barrel magnetic field; 5 a pincushion magnetic field. By adjusting deflecting amounts of the electron beams 1 and 3 on both sides, rasters of both of these electron beams, i.e., a red raster 6 and a blue raster 7 can be made to coincide as shown in FIG. 3. However, a deflecting amount of the central electron beam, i.e., the green electron beam 2 is slightly different from those of the electron beams on both sides. Therefore, a green raster 8 is formed in the inside or outside of the red raster 6 and blue raster 7. In FIG. 3, the case where the red raster 6 and blue raster 7 are formed in the inside of the green raster 8 is called a green narrow (N). Similarly, the case where they are formed in the outside of the green raster 8 is called a green wide (W). To make the green raster 8 coincide with the red raster 6 and blue raster 7, in general, hitherto, two kinds of magnetic material members 9 and 10 having different shapes for correction as shown in, e.g., FIG. 4 are arranged at electron beam emitting ends of the electron guns, thereby correcting a coma distortion.
For example, a magnetic material element to correct a coma distortion is disclosed in, for example, JP-B-No. 61-17094 filed in Japan by Toshiba Corporation on May 12, 1976, U.S. co-pending application Ser. No. 12,997 filed in the U.S.A. on Feb. 10, 1987 by ISHIBASHI et al of the same assignee as the present application, or U.S. Pat. No. 3,873,879 assigned to Hughes.
This magnetic material element tends to give a shielding effect to vertical and horizontal leakage magnetic fields 11 and 12 to the side of the electron guns of the deflecting coils. By changing the deflecting effects of the leakage magnetic fields to the red and blue electron beams (outer beams) and to the green electron beam (inner beam), those electron beams can be made to coincide on the screen.
However, the sizes and arranging positions of the two kinds of magnetic material elements having different sizes are not unconditionally determined by the magnitudes of the leakage magnetic fields or the sizes of the deflecting coils. It cannot help finding out the optimum combination by trial and error by preparing a plurality of combinations of several kinds of magnetic material elements of different sizes. Therefore, it takes a long time to determine the sizes and positions of two magnetic materials for obtaining the best coma distortion correcting effect.